1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a communications apparatus using the same.
2. Description of the Related Art
Recently, advances in the information and communications fields have led to increases in the amounts of transmitted information. Consequently, there is a demand for a relatively wide-band surface acoustic wave filter which excels in phase linearity within its pass band. A conventional filter which meets this demand is a transversal surface acoustic wave filter. However, as is well known, transversal surface acoustic wave filters have large insertion losses. Thus, as a technology for reducing insertion loss, expectations are placed on, and consideration is given to, transversal surface acoustic wave filters which employ unidirectional electrodes.
Now, conventional transversal surface acoustic wave filters which employ unidirectional electrodes will be described below. FIG. 22A shows a first prior art example of transversal surface acoustic wave filters. Reference numeral 2001 denotes a piezoelectric substrate. A surface acoustic wave filter is constructed by forming input and output interdigital transducer electrodes 2002 and 2003 (hereinafter referred to as IDT electrodes) on the piezoelectric substrate 2001, disposed at a certain distance from each other.
As the input IDT electrode 2002 and output IDT electrode 2003, first and second unidirectional electrodes 2004 and 2005 are used which are based on a prior art technology disclosed in xe2x80x9cIEEE Ultrasonics Symposium, 1989, pp. 77-89.xe2x80x9d This type of unidirectional electrode is known as an EWC-SPUDT (Electrode Width Controlled-Single Phase Unidirectional Transducer).
FIG. 22B shows an enlarged view of the first basic unidirectional-electrode cell 2004. The basic unit consists of three electrode fingers: a xcex/4-wide electrode finger one-fourth as long as the wavelength xcex of surface acoustic waves propagating on the piezoelectric substrate 2001 and two xcex/8-wide electrode fingers. The second basic unidirectional-electrode cell 2005 is bilaterally symmetrical to the first unidirectional electrode 2004.
In the first and second basic unidirectional-electrode cells 2004 and 2005, the xcex/4-wide electrode fingers function as reflectors. By placing the reflection center of the electrode fingers asymmetrically to the excitation center of surface acoustic waves, it is possible to give directivity to the propagation of the surface acoustic waves. The first basic unidirectional-electrode cell 2004 obtains directivity running from the input IDT electrode 2002 towards the output IDT electrode 2003 while the second basic unidirectional-electrode cell 2005 obtains directivity running from the output IDT electrode 2003 towards the input IDT electrode 2002.
FIG. 23 shows an electrode configuration disclosed in Japanese Published Unexamined Patent Application No. 2000-77973. FIG. 23A shows a basic section of an IDT electrode. It consists of a 3xcex/8-wide electrode finger and two xcex/8-wide electrode fingers. It is unidirectional and reflective in the right direction. The basic section here means a part of the IDT electrode which corresponds to one wavelength. The basic section of an electrode shown in FIG. 23B has excitation effect, but does not produce unidirectional reflections. Thus, it is bidirectional. In either case, the basic section is prescribed by the wavelength xcex. The basic section of an electrode shown in FIG. 23B has excitation effect, but substantially does not reflect waves.
FIG. 24 shows a second prior art example of surface acoustic wave filters using an electrode configuration disclosed in Japanese Published Unexamined Patent Application No. 2000-91869. In FIG. 24A, an input IDT electrode 2206 and output IDT electrode 2207 are mounted on a piezoelectric substrate 2201. The input IDT electrode 2206 consists of interdigitated electrode finger pairs 2202a and 2202b interlocked or engaged with each other in a staggered configuration and extraction electrodes 2204a and 2204b which connects individual pairs 2202a or 2202b while the output IDT electrode 2297 similarly consists of interdigitated electrode finger pairs 2203a and 2203b interlocked or engaged with each other in a staggered configuration and extraction electrodes 2205a and 2205b which connects individual pairs 2203a or 2203b. Each interdigitated electrode finger pair 2202a, 2202b, 2203a, or 2203b consists of two interdigitated electrode fingers of different line widths.
FIG. 24B shows an enlarged view of the input IDT electrode 2206 shown in FIG. 24A. If the spacing between the narrower interdigitated electrode finger and wider interdigitated electrode finger of an interdigitated electrode finger pair 2202b connected to the extraction electrode 2204b is denoted by xcex3, xc2xd the spacing between the narrower interdigitated electrode finger in the interdigitated electrode finger pair 2202b and the wider interdigitated electrode finger in the adjacent interdigitated electrode finger pair 2202a connected to the extraction electrode 2204a is denoted by xcex1, and xc2xd the spacing between the wider interdigitated electrode finger in the interdigitated electrode finger pair 2202b and the narrower interdigitated electrode finger in the adjacent interdigitated electrode finger pair 2202a connected to the extraction electrode 2204a is denoted by xcex2, then xcex3 greater than xcex1+xcex2.
With the unidirectional electrode shown in FIG. 24, if the width ratio of the wider interdigitated electrode finger to the narrower interdigitated electrode finger is defined as a line width ratio, the desirable range of the line width ratio from the viewpoint of directivity in relation to insertion loss is said to be no smaller than 3.0 and less than 5.0.
Recently, as portable terminals have become increasingly smaller and more sophisticated, smaller size and higher performance have been required of IF-stage surface acoustic wave filters. However, with the transversal surface acoustic wave filters employing any of the unidirectional electrodes described above, there is a difference in frequency between the region of a unidirectional electrode and the region of a bidirectional electrode, resulting in degradation of both in-band flatness and sharp out-of-band attenuation.
Also, with the transversal surface acoustic wave filter described above, the desirable range of the line width ratio, which is said to be no smaller than 3.0 and no larger than 5.0, is discussed only in terms of unidirectionality in relation to insertion loss, and any optimum electrode configuration which takes other elements into consideration is not suggested. Besides, actual surface acoustic wave filters often employ a combination of unidirectional and bidirectional electrodes, but no optimum electrode configuration is suggested for it either.
On the other hand, regarding the configuration consisting of only unidirectional electrodes, relationships with L2/L1 are not analyzed sufficiently.
One aspect of the present invention is a surface acoustic wave filter comprising a piezoelectric substrate and at least two interdigital transducer electrodes on the above described piezoelectric substrate,
wherein at least one of the above described interdigital transducer electrodes contains a unidirectional electrode and bidirectional electrode, and
the length xcexd of a basic cell of the above described unidirectional electrode is longer than the length xcexs of a basic cell of the above described bidirectional electrode.
Thanks to the above configuration, the present invention can provide a low-loss surface acoustic wave filter which excels in in-band flatness and attenuation characteristics.
Another aspect of the present invention is the surface acoustic wave filter wherein:
said unidirectional electrode has four electrode fingers in the basic cell prescribed by one wavelength and said four electrode fingers form two electrode finger pairs which are interdigitated with each other; and
adjacent electrode fingers in each of said two electrode finger pairs have different electrode widths and the electrode width ratio (L2/L1) of the wider electrode finger (L2) to the narrower electrode finger (L1) is larger than 1.
Still another aspect of the present invention is the surface acoustic wave filter wherein said xcexd/xcexs is determined such that the frequency of said unidirectional electrode and frequency of said bidirectional electrode will be substantially equal.
Yet still another aspect of the present invention is the surface acoustic wave filter wherein the film thickness of said unidirectional electrode and film thickness of said bidirectional electrode are substantially equal.
Still yet another aspect of the present invention is the surface acoustic wave filter wherein said bidirectional electrode has electrode fingers of equal width in the basic cell prescribed by one wavelength.
A further aspect of the present invention is the surface acoustic wave filter wherein a phase difference substantially equal to ((45+(nxe2x88x921)xc3x9790)) degrees, when n is a positive number, is provided between the excitation center and reflection center of an electrode finger pair in said unidirectional electrode by making the spacing between the narrower electrode finger and wider electrode finger in the electrode finger pair in said unidirectional electrode larger than the xe2x85x9 wavelength.
A still further aspect of the present invention is the surface acoustic wave filter wherein said electrode finger pair satisfies.
xcex3 greater than xcex1+xcex2
xcex1 less than xcex2xe2x80x83xe2x80x83(Formula 4) 
where:
xcex3 is the distance between the narrower and wider electrode fingers in the electrode finger pair
xcex1 is the distance between the narrower electrode finger and the left end of the segment obtained by dividing an IDT electrode into half-wavelengths
xcex2 is the distance between the wider electrode finger and the right end of the segment obtained by dividing the IDT electrode into half-wavelengths.
A yet further aspect of the present invention is the surface acoustic wave filter wherein xcex1 and xcex2 in the Formula 4 have been adjusted such that radiation characteristics of one of said interdigital transducer electrodes will be bilaterally symmetrical with respect to the center frequency.
A still yet further aspect of the present invention is the surface acoustic wave filter, wherein:
said unidirectional electrode has four electrode fingers in the basic cell prescribed by one wavelength and said four electrode fingers form two electrode finger pairs which are interdigitated with each other;
adjacent electrode fingers in each of said two electrode finger pairs have different electrode widths and the electrode width ratio (L2/L1) of the wider electrode finger (L2) to the narrower electrode finger (L1) is larger than 1; and
the electrode width ratio of the electrode finger pairs in said unidirectional electrode is in the range of 1.1xe2x89xa6L2/L1xe2x89xa64.2.
An additional aspect of the present invention is the surface acoustic wave filter, wherein the electrode width ratio of the electrode finger pairs in said unidirectional electrode is in the range of 1.1xe2x89xa6L2/L1xe2x89xa62.5.
A still additional aspect of the present invention is the surface acoustic wave filter, wherein at least one of the interdigital transducer electrodes contains a first unidirectional electrode for amplifying surface acoustic waves in one direction and a second unidirectional electrode for amplifying surface acoustic waves in the opposite direction.
A yet additional aspect of the present invention is the surface acoustic wave filter, wherein said first unidirectional electrode and said second unidirectional electrode are configured by exchanging the positions of the narrower electrode finger and wider electrode finger or by reversing the cell from left to right.
A still yet additional aspect of the present invention is the surface acoustic wave filter, wherein the electrode width ratio (L2/L1) of the electrode finger pairs in said unidirectional electrode is in the range of 1.1xe2x89xa6L2/L1xe2x89xa62.2.
A supplementary aspect of the present invention is the surface acoustic wave filter, wherein said interdigital transducer electrodes comprise unidirectional electrodes which have multi-directional intensities consisting of two or more electrode width ratios.
A still supplementary aspect of the present invention is the surface acoustic wave filter, wherein said piezoelectric substrate is a 28- to 42-degree rot-Y-cut quartz substrate.
A yet supplementary aspect of the present invention is a communications apparatus containing a surface acoustic wave filter.